Synthesizers utilized in known radio frequency (RF) communication devices, such as an RF transmitter, have included a voltage controlled oscillator and a reference oscillator coupled together in a phase locked loop. A modulation input is coupled to both the voltage controlled oscillator and the reference oscillator wherein the voltage controlled oscillator is responsive to high frequency modulation inputs and the reference oscillator is responsive to low frequency modulation inputs. In such systems, it is desirable that a given peak to peak voltage input causes a particular change in the frequency of the signal output from the voltage controlled oscillator. If a change in frequency that is greater than the desired change results, it is possible that the communication device will enter a different communication channel than the one desired. Alternatively, if the change in frequency is too low, it will not be accurately detected and decoded by the intended receiver. Further, inbalances in the high and low frequency modulation ports of the voltage controlled oscillator and reference oscillator can create modulation distortions.
In order to equalize the frequency deviation response produced by the voltage controlled oscillator modulation port and the reference oscillator modulation port, one known approach has included modifying the magnitude of the modulation inputs applied to the voltage controlled oscillator and the reference oscillator. However, although this approach is simple, it does not equalize phase differences present in the two modulation paths. This may result in unacceptable distortion in the frequency deviation response produced by the modulation ports.